U.S. patent application number 11/776360 was filed with the patent office on 2008-01-10 for systems and methods of blood-based therapies having a microfluidic membraneless exchange device.
This patent application is currently assigned to The Trustees of Columbia University in the City of New York. Invention is credited to Edward F. Leonard, Nina C. Shapley, Zhongliang Tang, Alan C. West.
Application Number | 20080009780 11/776360 |
Document ID | / |
Family ID | 33029896 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080009780 |
Kind Code |
A1 |
Leonard; Edward F. ; et
al. |
January 10, 2008 |
SYSTEMS AND METHODS OF BLOOD-BASED THERAPIES HAVING A MICROFLUIDIC
MEMBRANELESS EXCHANGE DEVICE
Abstract
The present invention is directed to devices, systems and
methods for removing undesirable materials from a sample fluid by
contact with a second fluid. The sample fluid flows as a thin layer
adjacent to, or between, concurrently flowing layers of the second
fluid, without an intervening membrane. In various embodiments, a
secondary separator is used to restrict the removal of desirable
substances and effect the removal of undesirable substances from
blood. The invention is useful in a variety of situations where a
sample fluid is to be purified via a diffusion mechanism against an
extractor fluid. Moreover, the invention may be used for the
removal of components from a sample fluid that vary in size. When
blood is the sample fluid, for example, this may include the
removal of `small` molecules, `middle` molecules, macromolecules,
macromolecular aggregates, and cells, from the blood sample to the
extractor fluid.
Inventors: |
Leonard; Edward F.;
(Scarsdale, NY) ; West; Alan C.; (Tenafly, NJ)
; Shapley; Nina C.; (New York, NY) ; Tang;
Zhongliang; (Hightstown, NJ) |
Correspondence
Address: |
PROSKAUER ROSE LLP
1001 PENNSYLVANIA AVE, N.W.,
SUITE 400 SOUTH
WASHINGTON
DC
20004
US
|
Assignee: |
The Trustees of Columbia University
in the City of New York
New York
NY
10027
|
Family ID: |
33029896 |
Appl. No.: |
11/776360 |
Filed: |
July 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10801366 |
Mar 15, 2004 |
|
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|
11776360 |
Jul 11, 2007 |
|
|
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60454579 |
Mar 14, 2003 |
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Current U.S.
Class: |
604/5.04 |
Current CPC
Class: |
B01D 11/0492 20130101;
B01D 11/0496 20130101; B01L 3/5027 20130101; A61M 1/14 20130101;
A61M 2206/11 20130101; B01D 11/0288 20130101 |
Class at
Publication: |
604/005.04 |
International
Class: |
A61M 1/34 20060101
A61M001/34 |
Claims
1. A method of performing a blood treatment, comprising: placing
blood from a patient in direct contact with an extraction fluid
such that blood proteins and uremic toxins move from the blood into
the extraction fluid; passing water and uremic toxins in the
extraction fluid through a membrane to concentrate the blood
proteins in a fraction of the extraction fluid and repeating the
placing the resulting fraction in direct contact with the blood
such that blood proteins are returned to the blood.
2. The method of claim 1, wherein the placing includes preventing
cells from moving from the blood into the extraction fluid.
3. The method of claim 1, wherein the placing includes flowing the
blood and extraction fluid through a channel with a ratio of width
(the dimension perpendicular to flow direction and parallel to the
interface between the blood and extraction fluid) and height (the
direction normal to the interface between the blood and extraction
fluid) is more than 10.
4. The method of claim 3, wherein the placing includes flowing the
blood and extraction fluid through a channel with a ratio of width
to height that is more than 50.
5. The method of claim 1, wherein the channel height is less than
100 microns.
6. The method of claim 1, wherein the extraction fluid includes
dialysate.
7. The method of claim 1, wherein the placing includes creating a
laminar flow of the blood and extraction fluids.
8. The method of claim 1, wherein the placing includes creating a
laminar flow of the blood and extraction fluids including two
layers of extraction fluid with a blood layer sandwiched between
them.
9. The method of claim 1, wherein the placing includes creating a
laminar flow of the blood and extraction fluids including two
layers of extraction fluid with a blood layer sandwiched between
them; the flow being created in a channel with a ratio of width
(the dimension perpendicular to flow direction and parallel to the
interface between the blood and extraction fluid) to height (the
direction normal to the interface between the blood and extraction
fluid) of more than 10 and with a height of less than 100
microns.
10. The method of claim 1, wherein the placing includes creating a
laminar parallel flow of the blood and extraction fluids including
two layers of extraction fluid with a blood layer sandwiched
between them, the total volume flow rate of blood and the total
volume flow rate of extraction fluid being approximately the
same.
11. The method of claim 1, wherein the placing includes creating a
laminar flow of the blood and extraction fluids including two
layers of extraction fluid and a blood layer sandwiched between
them, the total volume flow rate of blood in the blood layer and
the total volume flow rate of extraction fluid in the two
extraction fluid layers being approximately the same.
12. The method of claim 1, wherein the passing includes circulating
the extraction fluid across a single side of a membrane without
passing it through the membrane such that the resulting fraction is
depleted of water and uremic toxins.
13. The method of claim 1, wherein the passing includes pumping the
extraction fluid using a first pump and pumping the blood using a
second pump.
14. The method of claim 1, wherein the placing includes flowing the
blood and extraction fluid to form a flat blood layer in which
blood cells tend to drift toward a center of the blood layer.
15. The method of claim 1, wherein the placing includes flowing the
blood and extraction fluid to form a flat blood layer and at least
one extraction fluid layer, the combined blood layer and at least
one extraction fluid layer defining a velocity profile in which the
blood layer coincides with a region of minimum shear rate such that
cells tend to remain in the blood layer as a result of a tendency
of cells to migrate away from high shear rate regions of a
flow.
16. The method of claim 1, wherein the placing includes creating,
in a channel with walls, a laminar parallel flow between the walls
that includes two layers of extraction fluid with a blood layer
sandwiched between them, the flow being such that the cells within
the blood layer do not contact the walls.
17. A method of performing a blood treatment, comprising:
establishing a flow of blood and extraction fluid in a channel such
that the blood and extraction fluid are in direct contact and such
that entering and exiting flows of each of the blood and extraction
fluid into and from the channel are established, respectively, at
opposing ends of the channel; conveying a portion of the extraction
fluid in the extraction fluid exiting flow to the extraction fluid
entering flow such that a quantity of blood proteins leaving the
channel in the extraction fluid exiting flow is substantially equal
to the quantity of blood proteins returned to the channel entering
flow.
18. The method of claim 17, wherein the portion of the extraction
fluid in the extraction fluid exiting flow conveyed to the
extraction fluid entering flow is obtained by removing water and
uremic toxins from the extraction fluid exiting flow by passing the
water and uremic toxins through a membrane.
19. The method of claim 18, wherein the establishing includes
retaining blood cells in the blood exiting flow and preventing them
from leaving in the extraction fluid exiting flow by maintaining a
lower shear rate at a location of the channel coinciding with the
blood exiting flow than a shear rate at one or more locations of
the channel coinciding with the extraction fluid exiting flow.
20. A method of performing a blood treatment, comprising: flowing
blood and dialysate into a microfluidic channel such that the blood
and dialysate are in direct contact but remain in separate layers
in the channel; the channel having a width to height ratio of more
than 10; the flowing being such that a lower shear rate is
maintained in the blood layer than a shear rate maintained in one
or more dialysate layers, the difference in shear rate being
sufficient to cause blood cells to be retained in the blood layer
while permitting blood proteins and uremic toxins to diffuse into
the one or more dialysate layers; passing dialysate exiting the
channel across one side of a membrane such that water and uremic
toxins flow through the membrane and out of the dialysate while
preventing blood proteins from passing through the membrane thereby
retaining the blood proteins in the dialysate and thereafter
returning the dialysate and blood proteins back to the channel.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The application is a divisional of U.S. application Ser. No.
10/801,366, filed Mar. 15, 2004, pending, which claims the benefit
of U.S. Provisional Application Ser. No. 60/454,579, filed Mar. 14,
2003, expired, both of which are hereby incorporated by reference
in their entireties.
FIELD OF THE INVENTION
[0002] Generally speaking, the present invention relates to the
purification of a sample fluid. More particularly, the present
invention relates to the purification of a sample fluid, blood
fluid) by selectively removing components using a microfluidic
membraneless exchange device.
BACKGROUND OF THE INVENTION
[0003] Extracorporeal processing of blood is known to have many
uses. Such processing may be used, for example, to provide
treatment of a disease. Hemodialysis is the most commonly employed
form of extacorporeal processing for this purpose. Additional uses
for extracorporeal processing include extracting blood components
useful in either treating others or in research. Apheresis of
plasma (i.e., plasmaphesis) and thrombocytes, or platelets, are the
procedures most commonly employed for this purpose.
[0004] Many different extracorporeal blood processing processes
have been developed, each of which seeks to remove certain
components from the blood, depending on the reason for processing
the blood. (It will be understood that as used herein, blood, or
blood fluid, refers to any fluid having blood components from which
extraction of certain components, such as toxins or albumin, is
desired.) The most common process utilizes an artificial membrane
of substantial area, across which selected blood components are
induced to flow. This flow is generally induced by a transmembrane
difference in either concentration or pressure, or a combination of
the two. Another form of blood processing calls for the separation
of certain components from blood by passing the blood over sorbent
particles. In yet other forms of blood processing, not practiced as
commonly, blood is directly contacted with an immiscible liquid
(e.g., a fluorocarbon liquid), with the desired result being the
removal of dissolved carbon dioxide and the provision of oxygen.
The usefulness of blood processing techniques employing immiscible
liquids is limited, however, because these immiscible liquids
generally have very limited capacity to accept the blood components
that it is desirable to extract.
[0005] One common example of a therapeutic use for blood processing
is the mitigation of the species and volume imbalances accompanying
end-stage renal disease. The population of patients treated in this
manner (i.e., through hemodialysis) exceeds 260,000 and continues
to grow, with the cost of basic therapy exceeding $5 billion per
year excluding complications. The overwhelming majority of these
patients (about 90%), moreover, are treated in dialysis centers,
generally in thrice-weekly sessions. While procedures have
been--and continue to be--refined, the components and the geometry
of hemodialysis were largely fixed in the 1970's: a bundle of
several thousand, permeable hollow fibers, each about 25 cm long
and about 200 .mu.m internal diameter, perfused externally by
dialyzing solution, with the device operated principally in a
diffusive mode but with a transmembrane pressure applied to induce
a convective outflow of water. Upward of 120 liters per week of
patient blood are dialyzed against upwards of 200 liters per week
of dialyzing solution, often in three weekly treatments that total
as little as seven to nine hours per week. These numbers vary
somewhat, and competing technologies exist, but the basic approach
just described predominates.
[0006] Despite the benefits of therapies (e.g., hemodialysis) using
the various forms of blood processing described above, the
prolongation of life achieved is complicated by the progression and
complexity of the disease the therapies are used to treat (few
patients on dialysis are ever completely rehabilitated), and by
several problems that are innate to the therapies themselves. For
example, problems arise with blood processing as a result of the
contact of blood with extensive areas of artificial membrane (as in
the case of hemodialysis), and well as the contact of blood with
sorbents or immiscible fluids as described above. In particular,
this contact often induces biochemical reactions in the blood being
processed, including the reactions that are responsible for
clotting, activation of the complement systems, and irreversible
aggregation of blood proteins and cells.
[0007] Another problem associated with known blood processing
techniques is that the contact of blood with an artificial membrane
(or another medium, such as a sorbent or immiscible fluid) is
likely to cause the blood-medium interface to become fouled. It is
generally known that therapeutic interventions (e.g., those related
to end-stage renal disease) are optimally conducted with slow
delivery and in as nearly a continuous fashion as possible, in
emulation of the continuous action of a natural kidney. However,
fouling caused by the contact of blood with the medium limits the
time that a device which contains these interfaces can be usefully
employed. As a result, portable blood processing devices become
impractical, and patients are generally forced to undergo the type
of episodic dialysis schedule described above, which creates many
negative side effects such as physical exhaustion and excessive
thirst. Moreover, even while daily dialysis (e.g., 1.5-2.0 hours,
six days per week) or nocturnal dialysis (e.g., 8-10 hours, 6-7
nights per week) improves this situation by extending treatment
times, a patient using one of these forms of treatment is still
required to remain near a hospital or clinical facility that can
administer the dialysis procedure.
[0008] In light of the above, it would be desirable to provide
techniques for processing blood in which treatment times are
extended (with consequently lower rates of flow) and that do not
require a patient to remain near a hospital or clinic. Moreover, it
would also be desirable to provide techniques for processing blood
that eliminate (or at least reduce) the inducement of undesirable
biochemical reactions, and where the blood-medium interfaces do not
become fouled.
SUMMARY OF THE INVENTION
[0009] The above and other deficiencies associated with existing
blood processing processes are overcome in accordance with the
principles of the present invention which are described below.
According to one aspect of the invention, a membraneless exchange
device for extracting components from a sample fluid is described
which includes first, second and third inlet channels, first,
second and third exit channels and a microfluidic extraction
channel connected to the first, second and third inlet channels and
the first, second and third exit channels. Moreover, laminar flows
of a first extractor fluid, the sample fluid, and a second
extractor fluid are established inside the extraction channel, and
sheathing of the sample fluid by the first and second extractor
fluids substantially limits contact between the sample fluid and
the surfaces of the extraction channel.
[0010] According to another embodiment of the present invention, a
system for performing hemodialysis is provided which includes a
membraneless exchange device including first and second dialysate
inlet channels, blood inlet and exit channels, first and second
dialysate exit channels and a microfluidic dialysis channel
connected to the first and second dialysate inlet and outlet
channels and the blood inlet and exit channels. Moreover, laminar
flows of a first dialysate fluid, blood fluid, and a second
dialysate fluid are established in order inside the dialysis
channel, and at least some of the components of the blood fluid
exits the device through the first and second dialysate exit
channels. Additionally, according to the invention, a secondary
processor receives the dialysate fluid and the at least some of the
components of the blood fluid exiting the device through the first
and second dialysate exit channels.
[0011] In yet another embodiment of the present invention, a method
for extracting components from a sample fluid is provides which
includes establishing laminar flows of a first extractor fluid, the
sample fluid and a second extractor fluid inside a microfluidic
extraction channel. Sheathing of the sample fluid by the first and
second extractor fluids, moreover, substantially limits contact
between the sample fluid and the surfaces of the extraction
channel. The method further includes withdrawing the first
extractor fluid, the sample fluid and the second extractor fluid
from the extraction channel such that at least a portion of the
sample fluid is removed together with the first extractor fluid and
the second extractor fluid and apart from the remainder of the
sample fluid.
[0012] A method for performing hemodialysis is also provided which
includes establishing laminar flows of a first dialysate fluid,
blood fluid and a second dialysate fluid inside a microfluidic
extraction channel, withdrawing the first dialysate fluid, the
blood fluid and the second dialysate fluid from the extraction
channel such that at least some of the components of the blood
fluid are removed together with the first dialysate fluid and the
second dialysate fluid and apart from the remainder of the blood
fluid, and providing the first and second dialysate fluids and the
at least some of the components of the blood fluid to a secondary
processor.
[0013] In general, however, the present invention is directed
toward microfluidic membraneless exchange devices and systems, and
methods of making the same, for selectively removing undesirable
materials from a sample fluid (e.g., blood fluid) by contact with a
miscible fluid (extractor fluid or secondary fluid, e.g.,
dialysate). A microfluidic device, as considered in this
application, has channels whose height is less than about 0.6 mm,
where "height" is the dimension perpendicular to the direction of
flow and also perpendicular to the interfacial area across which
transport occurs. For example, flow patterns and species exchanges
occur when blood is flowed as a thin layer adjacent to, or between,
concurrently flowing layers of a secondary fluid, without an
intervening membrane. The secondary fluid, moreover, is generally
miscible with blood and diffusive and convective transport of all
components is expected. The following reference which refers to
membraneless devices described below is hereby incorporated by
reference in its entirety: Leonard et al., Dialysis without
Membranes: How and Why?, Blood Purification 22 (1) 2004 92-100.
[0014] Sheathing a core of blood with the miscible fluid, or
assuring that the miscible fluid lies between at least a
substantial portion of the blood and the enclosing boundaries of
the flow path, prevents or at least limits contact of the blood
with these boundaries. In turn, this configuration of the two
fluids prevents or at least reduces the undesirable activation of
factors in the blood, thereby minimizing bioincompatibilities that
have been problematic in prior techniques of blood processing.
[0015] The invention also eliminates or at least substantially
reduces the fouling reactions that have been known to be a major
deterrent to the continuous use of an extracorporeal extraction
device. In particular, as the primary transport surface in the
membraneless exchange device (also referred to herein as a
membraneless separator) of the invention is intrinsically
non-fouling, a major deterrent to long-term or continuous operation
is removed, opening the possibility to the design and construction
of small, wearable devices or systems with the recognized benefits
of nearly continuous blood treatment. Such a device or system could
be very small and worn or carried by the patient (e.g., outside of
a hospital or clinic setting), and could be supplied with external
buffer reservoirs (in a back-pack, briefcase, or from a reservoir
located in the home, located at the place of work, etc.). Further,
because fouling would be reduced, and sustained operation at low
blood flows over long times would be allowed, such anticoagulation
as might be required is likely to have an effect confined to the
extracorporeal circuit. As understood by those skilled in the art,
avoiding systemic anticoagulation outside of the clinic is highly
desirable.
[0016] The devices, systems and methods of the invention described
herein also have the benefit of being capable of diffusing various
blood components having different sizes. In particular, the flow of
blood and a miscible fluid with which it is in contact can be
controlled for the purpose of achieving the desired separation of
components (e.g., separating molecules of low molecular weight
only). For example, as explained below, various flow conditions may
be used that cause blood cells to move away from the blood-liquid
interface, thereby making it is possible to "skim" blood in order
to remove substantial amounts of plasma, without cells.
[0017] As also discussed below, membraneless contact of a thin
layer of blood with a sheathing fluid according to the present
invention may be used to cause high rates of exchange per unit area
of blood-sheathing fluid contact for all solutes, but with a
discrimination among free (unbound) solutes that is less than the
square-root of the ratio of their diffusion coefficients. Moreover,
while high exchange rates (e.g., of toxic substances) are often
desirable, indiscriminate transport is not. Therefore, according to
the principles of the present invention, a membraneless exchange
device as described herein is used in conjunction with at least one
secondary processor (e.g., a membrane device or other type of
separator) in order to restrict the removal of desirable substances
and effect the removal of undesirable substances from blood. The
efficiency of such a secondary separator is greatly increased by
the use of the primary separator that is capable of delivering
cell-depleted (or cell-free) fractions of blood to it. Therefore,
according to another aspect of this invention, transport of
molecular components of blood to the sheathing fluid may be
indiscriminate. The sheathing fluid, carrying both those molecular
components which it is, and is not, desirable to remove from blood,
is provided to the secondary separator, such that the fluid
entering the secondary separator is substantially cell-free. The
secondary separator, meanwhile, regulates the operation of the
membraneless separator through the composition of the recycle
stream that it returns (directly or indirectly) to the sheath fluid
inlets of the membraneless separator. According to the principles
of the present invention, moreover, a membrane-based secondary
separator used in this manner is able to achieve much higher
separation velocities because concentration polarization (i.e., the
accumulation of material rejected by the secondary separator on the
upstream side of the separator) is limited to proteins and does not
involve cells. Moreover, because cells would be retained in the
primary separator (i.e., the membraneless exchange device), they
would see artificial material only on its conduit surfaces, not on
its liquid-liquid contact area, whence bioincompatibilities should
be much reduced. As such, it should be understood that the need for
anticoagulation may be greatly reduced or eliminated.
[0018] Further features of the invention, its nature and various
advantages, will be more apparent upon consideration of the
following detailed description, taken in conjunction with the
accompanying drawings, in which like reference characters refer to
like parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows the velocity profile of a core stream of blood
sheathed on both of its sides by a dialysate fluid calculated for
blood with a viscosity assumed twice that of the dialysate fluid
and with a centerline velocity of 5 cm/sec.
[0020] FIG. 2 shows a plot using Loschmidt's formula of 1870, where
each fluid layer has the same thickness.
[0021] FIG. 3 shows a simplified view of a membraneless separator
constructed in accordance with the principles of the present
invention.
[0022] FIG. 4 shows membraneless separator used for the purpose of
plasmapheresis in accordance with the principles of the present
invention.
[0023] FIG. 5 shows the image of a portion of the membraneless
separator of FIG. 5 while plasma is being skimmed from blood, as
obtained by using a CCD camera.
[0024] FIG. 6 shows a simplified block diagram of a system
including a membraneless separator and a secondary separator in
accordance with the principles of the present invention.
[0025] FIG. 7 shows a more detailed view of a system including
primary and secondary separators in accordance with the principles
of the present invention.
[0026] FIG. 8 shows the configuration of a system subdivided into
three units in accordance with the principles of the present
invention.
[0027] FIG. 9 shows the routing of fluids between separate units in
accordance with the principles of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] According to one aspect of the invention, a membraneless
exchange device for extracting components from a sample fluid is
described which includes first, second and third inlet channels,
first, second and third exit channels and a microfluidic extraction
channel connected to the first, second and third inlet channels and
the first, second and third exit channels. Moreover, laminar flows
of a first extractor fluid, the sample fluid, and a second
extractor fluid are established inside the extraction channel, and
sheathing of the sample fluid by the first and second extractor
fluids substantially limits contact between the sample fluid and
the surfaces of the extraction channel. In one embodiment of the
device, at least 90% of the sample fluid is sheathed by the first
and second extractor fluids. In other embodiments, 95% of the
sample fluid is sheathed. In yet other embodiments, at least a
portion of the sample fluid exits the device with the first
extractor fluid through the first exit channel, and advective
transport of molecules within said extraction channel is
substantially nonexistent. The composition of the first extractor
fluid, moreover, is substantially the same as the composition of
the second extractor fluid is various embodiments. In other
preferred embodiments, the sample fluid flow is between the first
and second extractor fluid flows. Moreover, a first diverter is
formed from a portion of the first exit channel and a portion of
the second exit channel, while a second diverter is formed from a
portion of the second exit channel and a portion of the third exit
channel. It should also be understood that the device may include a
first interface formed between the first extractor fluid flow and
the sample fluid flow that is aligned with at least a portion of
the first diverter, and may also include a second interface formed
between the second extractor fluid flow and the sample fluid flow
that is aligned with at least a portion of the second diverter. In
various embodiments of the invention, moreover, the sample fluid is
blood fluid, in which case it is contemplated that the components
extracted from the sample fluid are non-cellular components of the
blood fluid. Additionally, the device may use a first pump for
controlling the flow of extractor fluid in the extraction channel,
and may use a second pump for controlling the flow of sample fluid
in the extraction channel. When a first pump is used, it may be an
injection pump that controls the flow of extractor fluid into the
extraction channel, and a withdrawal pump may be used that controls
the flow of extractor fluid out of the extraction channel. In
various embodiments, additionally, a source of extractor fluid is
connected to said first inlet channel and a source of sample fluid
connected to said second inlet channel. It will be understood that
the source of sample fluid can be, for example, a human being. In
preferred embodiments, moreover, the extraction channel of the
device according to the invention has a height of less than 600
.mu.m, and has a width-to-height ratio of at least ten. The device
may also be used in a system for extracting components from a
sample fluid, where the system also includes a secondary processor
that receives the first extractor fluid, the second extractor fluid
and at least some of the components of the sample fluid upon
exiting the extraction channel. It will be understood that the
secondary processor may be, for example, a membrane device or a
sorption device.
[0029] According to another embodiment of the present invention, a
system for performing hemodialysis is provided which includes a
membraneless exchange device including first and second dialysate
inlet channels, blood inlet and exit channels, first and second
dialysate exit channels and a microfluidic dialysis channel
connected to the first and second dialysate inlet and outlet
channels and the blood inlet and exit channels. Moreover, laminar
flows of a first dialysate fluid, blood fluid, and a second
dialysate fluid are established in order inside the dialysis
channel, and at least some of the components of the blood fluid
exits the device through the first and second dialysate exit
channels. Additionally, according to the invention, a secondary
processor receives the dialysate fluid and the at least some of the
components of the blood fluid exiting the device through the first
and second dialysate exit channels. In various embodiments, the
secondary processor filters the dialysate fluid and the at least
some of the components of the blood fluid exiting the device
through the first and second dialysate exit channels, and returns
the filtered fluid to the first and second dialysate inlet
channels. In certain preferred embodiments, these components of the
blood fluid are substantially non-cellular components of the blood
fluid. In other embodiments, sheathing of the blood fluid by the
first and second dialysate fluids substantially limits contact
between the blood fluid and the surfaces of the dialysis channel.
Moreover, the secondary processor may be a membrane device, or may
be a sorption device, for example. It will also be understood that
the composition of the first dialysis fluid may be substantially
the same as the composition of the second dialysis fluid. According
to other aspects of the invention, meanwhile, a first diverter is
formed from a portion of the first dialysate exit channel and a
portion of the blood exit channel, and a second diverter is formed
from a portion of the blood exit channel and a portion of the
second dialysate exit channel. A first pump for controlling the
flow of dialysate fluid in the dialysis channel and a second pump
for controlling the flow of blood fluid in the dialysis channel may
also be used in accordance with the principles of the present
invention. According to several embodiments, the interface between
the first dialysate fluid and the blood fluid is varied by
adjusting the velocities of the laminar flows of the first
dialysate fluid and the blood fluid. In other embodiments, the
interface between the blood fluid and the second dialysate fluid is
varied by adjusting the velocities of the laminar flows of the
blood fluid and the second dialysate fluid. A reservoir for storing
a viscosity agent may also be used in the system, where the
viscosity agent is mixed with the first and second dialysate fluid
to alter the viscosity of the first and second dialysate fluid.
Additionally, a detector for detecting a presence of an undesired
blood component within the dialysate fluid upon exiting the
dialysis chamber may be used. In this case, for example, the
detector is a photo detector. According to another aspect of the
invention, a first pump for controlling the flow of dialysate fluid
in the dialysis channel is adjusted based on said detected presence
of an undesired blood component within said dialysate fluid.
Moreover, for example, the velocities of the laminar flows of the
first dialysate fluid, the blood fluid and the second dialysate
fluid are adjusted based on the detected presence of an undesired
blood component within the first and second dialysate fluids
according to the invention. Additionally, according to the
invention, the first and second dialysate fluids may include at
least one of the following: a hyper osmolar solution, a solution
high in glucose content, or a polyelectrolye osmotic agent.
[0030] In yet another embodiment of the present invention, a method
for extracting components from a sample fluid is provides which
includes establishing laminar flows of a first extractor fluid, the
sample fluid and a second extractor fluid inside a microfluidic
extraction channel. Sheathing of the sample fluid by the first and
second extractor fluids, moreover, substantially limits contact
between the sample fluid and the surfaces of the extraction
channel. The method further includes withdrawing the first
extractor fluid, the sample fluid and the second extractor fluid
from the extraction channel such that at least a portion of the
sample fluid is removed together with the first extractor fluid and
the second extractor fluid and apart from the remainder of the
sample fluid. According to the invention, moreover, establishing
laminar flows includes providing first, second and third inlet
channels and providing first, second and third exit channels.
Additionally, for example, the method includes providing the first
and second extractor fluids and the at least a portion of the
sample fluid to a secondary processor.
[0031] A method for performing hemodialysis is also provided which
includes establishing laminar flows of a first dialysate fluid,
blood fluid and a second dialysate fluid inside a microfluidic
extraction channel, withdrawing the first dialysate fluid, the
blood fluid and the second dialysate fluid from the extraction
channel such that at least some of the components of the blood
fluid are removed together with the first dialysate fluid and the
second dialysate fluid and apart from the remainder of the blood
fluid, and providing the first and second dialysate fluids and the
at least some of the components of the blood fluid to a secondary
processor. In various embodiments, the method also includes using
the secondary processor to filter the first and second dialysate
fluids and the at least some of the components of the blood fluid,
as well as returning the filtered fluid from the secondary
processor to the extraction channel. In yet other embodiments, the
method includes sheathing the blood fluid by the first and second
dialysate fluids to substantially limit the contact between the
blood fluid and the surfaces of the dialysis channel.
[0032] Referring to FIG. 1, calculated for blood with a viscosity
assumed twice that of the sheathing fluid and with a centerline
velocity of 5 cm/sec, a flow path length of 10 cm would result in a
contact time of slightly longer than 2 sec. The steady contact of
two moving liquids for an exposure time determined by the length of
their contact area divided by their interfacial velocity
(.tau.=L/v) is highly analogous to the sudden exposure of one
volume of stagnant fluid to another for a specified time. Thus,
what happens to the flowing fluids along their shared flow path is
comparable to what would happen to two stagnant fluids over their
exposure time to each other. The stagnant fluid problem was solved
by Loschmidt in 1870. E = 1 2 - 4 .pi. 2 .times. 0 .infin. .times.
.times. 1 ( 2 .times. n + 1 ) 2 .times. exp .function. [ - ( 2
.times. n + 1 ) 2 .times. ( .pi. 2 .times. B ) 2 .times. D .times.
.times. t ] ##EQU1##
[0033] for which the zeroth order term, E = 1 2 - 4 .pi. 2 .times.
exp .function. ( - [ .pi. 2 .times. B ] 2 .times. D .times. .times.
t ) , ##EQU2## suffices when ( .pi. 2 .times. B ) 2 .times. D
.times. .times. t > 0.7 . ##EQU3##
[0034] This formula greatly simplifies the estimation of how much
mass can be transferred between fluids in a membraneless system. In
particular, this formula provides an approximation of the
extraction E of a component with a diffusion coefficient D when two
liquids flow side-by-side and remain in contact for an interval of
time, t.
[0035] FIG. 2, meanwhile, shows a plot using a version of
Loschmidt's formula, where each fluid layer has the same thickness
B (i.e., B is the half-thickness of the sheathed layer of sample
fluid). The situation shown in the plot of FIG. 2 can be
interpreted as a blood layer, of thickness B, contacting a layer of
sheathing fluid (i.e., extractor fluid). The sheathing layer is
presumed to be at zero concentration and E is the fraction of
material in the blood layer that is extracted in a time t, where D
is the diffusion coefficient of the extracted substance. If a layer
of thickness twice B is bounded on both sides by fluid layers of
thickness B, the formula still applies, as written. As indicated by
this formula, E cannot exceed 1/2 since the prescription of
concurrent flow allows, at best, the two fluids to come to
equilibrium.
[0036] For example, if one prescribes 90% of maximum extraction
(E=0.45), the ratio Dt/B.sup.2 must be approximately 0.86. Any
combination of diffusivity, layer thickness, and exposure time that
produces this value will produce the same extraction. Moreover, it
can be shown that the necessary area (2LW) to achieve this
extraction equals 0.86 BQ/D, where Q is the blood (and sheath
fluid) flow rate. Thus, for urea (D=10.sup.-5 cm.sup.2/sec) at a
blood flow rate of 0.3 cm.sup.3/sec, the required area is 2.57 B
10.sup.4 cm.sup.2. If B is taken to be 100 .mu.m, the required area
is 257 cm.sup.2. This flow corresponds to what might be needed in a
wearable artificial kidney. If, instead, a conventional flow of 5
cm.sup.3/sec were used, the required area would be 4300 cm.sup.2.
Thinner films, moreover, would require less area but would result
in higher shear rates and pressure gradients. In terms of
extraction, any combination of length L and width W that produces
the requisite area is equivalent. (If one assumes D for albumin to
be 5 10.sup.-7 cm.sup.2/sec, its extraction would be 0.116, 26% of
that for urea, unchangeable at this extraction level for urea).
[0037] It should be noted that use of the Loschmidt formula with
flowing systems introduces an incongruity that prevents precise
estimation of mass transfer rates and clearances, given that it
presumes that both fluids are moving at uniform velocity. In
particular, it provides an excellent approximation for the sheathed
fluid (blood), but ignores the nearly linear decay in velocity with
distance from the interface in the sheathing fluid. Nevertheless,
the Loschmidt formula is adequate for design purposes when the
sheathing layer has a total thickness (2B) that is twice that of
its half of the blood layer (as shown in FIG. 1), and thus a rate
of flow nearly equal to its half of the central stream.
[0038] The shear-induced self-diffusion coefficient of cells,
meanwhile, can be estimated by using the expression of Leighton and
Acrivos (1987) for concentrated suspensions: D.sub.particle.varies.
.phi..sup.2 .alpha..sup.2 {dot over (.gamma.)}.sup.2, where .phi.
is the particle volume fraction, .alpha. is the particle radius,
and {dot over (.gamma.)} is the shear rate. Then, the
characteristic displacement of a cell can be expressed as
.DELTA.y.varies. {square root over (D.sub.particlet)}. Choosing
representative values for the layered flow system such that the
cell volume fraction .phi. .apprxeq.0.45/2=0.225, the average
radius a of the red blood cell .apprxeq.2.5 .mu.m, and the average
shear rate {dot over (.gamma.)} over the blood layer.apprxeq.3 to
28 s.sup.-1 (based on an average velocity range of 0.5 to 5 cm/s),
we calculate that D.sub.particle.about.10.sup.-8 cm.sup.2/s, which
is approximately three orders of magnitude smaller than the typical
diffusion coefficient of small solutes. Based on this value of the
shear-induced diffusion coefficient (and assuming 10 sec of contact
between layers), it is estimated that blood cells are displaced by
a characteristic distance .DELTA.y.apprxeq.3 .tau.o 9 .mu.m from
the central layer, depending on the choice of blood velocity and
the concomitant shear rate. As explained in greater detail below,
this low distance of cell migration away from the central layer
facilitates the removal of cell-free portions of blood by the
membraneless separators described herein.
[0039] It should be noted that, according to one aspect of the
present invention, the removing of undesirable materials from a
sample fluid occurs under conditions that prevent advective mixing
of blood and the secondary fluid. In its general usage herein,
advection is used to describe the transport of fluid elements from
one region to another, and is used to distinguish disordered
convection from diffusion unaided by convection or diffusion in the
presence of only ordered and unidirectional convection. The term
advection is therefore used to mean a form of transport within a
fluid or between two contacting miscible fluid in which clumps of
fluid from two different positions are effectively interchanged.
Advection, so defined, can occur in turbulent flows or in unstable
laminar flows. Advective mixing, moreover, is often purposefully
induced by the application of a moving agitator blade to a fluid.
The prevention of advective mixing and the short contact times that
lead to small areas of contact (and, in turn, to a small device
that has a small size and a limited extracorporeal blood volume) is
greatly facilitated by the use of a microfluidic geometry. An
increase in channel height raises requisite contact time and tends
to reduce the stability of the sheathed flow. When total blood
layer thickness is 25, 50, or 100 .mu.m, and the blood flow is 20
ml/min (as it might be with a wearable artificial kidney), the
interfacial area needed to cause a substance such as urea
(D=10.sup.-5 cm.sup.2/sec) to reach 90% of equilibrium is,
respectively, 18, 36, and 71 cm.sup.2.
[0040] As mentioned above, the devices, systems and methods of the
present invention allow the purification of blood without the use
of a membrane by contact of the blood with a miscible fluid under
conditions that prevent advective mixing. It will be clear from the
detailed description of various embodiments of the invention
provided below that the invention is useful in hemodialysis, for
example. However, it should also be noted, and understood by those
skilled in the art, that the present invention is also useful in
other situations where a sample fluid is to be purified via a
diffusion mechanism against another fluid (e.g., an extractor
fluid).
[0041] According to the principles of the present invention, the
purification techniques described herein enable the flow of blood,
completely or partially surrounded by another liquid (e.g.,
extractor fluid) such that the streams are contacted in a small
channel and are subsequently separated at the end of the channel.
The middle stream is, thus, the blood to be purified, while the
surrounding stream (or streams) is the extractor fluid. This
membraneless contact, or sheathing of blood with layers of a
miscible fluid, according to principles of the present invention,
may occur along a flow path whose cross-section is either
rectangular, preferably of great breadth and limited thickness, or
circular. The invention is not limited in this manner.
[0042] Persons skilled in the art will appreciate that the
requisite transport areas, moreover, can be achieved by
combinations of channel length, width, and number according to the
principles of the present invention. In particular, Area=2 (top and
bottom).times.width.times.length.times.number of channels stacked
or otherwise arrayed in parallel. (As used herein, the term "width"
refers to a dimension perpendicular to the direction of flow and
parallel to the interface between the two liquids, while, as
explained above, the term "height" refers to a dimension
perpendicular to the direction of flow and also perpendicular to
the interface between the two fluids). It is shown herein that the
competing requirements of small height (to avoid excessive
diffusion times and in-process volumes), short length (to avoid
excessive pressure drop) and practical limitations on width of a
single device, which suggests the need to array them in parallel,
side-by-side or in a stack can be satisfied in practical
microfluidic devices.
[0043] FIG. 3 shows a simplified view of a membraneless separator
300 fabricated in flat-sheet configuration in accordance with the
principles of the present invention. According to one embodiment of
the present invention, three flat strips of copper foil, each three
centimeters wide, four centimeters long and 100 microns thick, are
soldered in their mid-sections to form extraction channel 302. The
ends (one centimeter) of the outer pieces are bent 30 degrees
outward to form three separate inlet channels 304, 306 and 308 and
three corresponding exit channels 310, 312 and 314 as shown in FIG.
3. According to the invention, the pieces are then coated with a
mold release agent, and the channel is then placed in a Petri dish.
At this time, an amount of PDMS precursor/curing agent mixture
(10:1 ratio), sufficient to form a two centimeter-thick polymer
layer after curing, is poured into the dish. After curing, the foil
assembly is easily released from the PDMS replica, and the replica
is sandwiched between two partially cured flat pieces of PDMS and
annealed to form a well-sealed channel. Finally, slight vacuum is
applied during the annealing to remove air bubbles trapped between
the flow channel module and the flat pieces, and the sealed
separator 300 is then ready for use (preferably after the chip is
rinsed with ethanol and with de-ionized water, and then dried with
compressed nitrogen gas). A flat piece of PDMS which served as a
cover to seal the chip by adhesion is also preferably cleaned and
dried in the same manner.
[0044] It will be understood that the particular fabrication
process described above is for purposes of illustration only. For
example, the dimensions of membraneless separator 300 may be
altered without departing from the scope of the present invention.
Additionally, for example, it will be understood that the invention
is not limited to the use of copper foil, and that other
fabrication processes not described may also be employed.
[0045] FIG. 4 shows a membraneless separator 400 according to the
principles of the present invention. Similar to separator 300
described above, separator 400 includes an extraction channel 402,
three separate inlet channels 404, 406 and 408 and three
corresponding exit channels 410, 412 and 414. As also shown in FIG.
4, a first diverter 416 is formed from portions of exit channels
410 and 412, while a second diverter 418 is formed from portions of
exit channels 412 and 414. It will be understood, however, that the
invention is not limited by the number of exit channels (or inlet
channels) that are used, nor is the invention limited by the number
of diverters formed therefrom.
[0046] As illustrated in FIG. 4, membraneless separator 400 can be
used as a plasmapheresis device in accordance with the principles
of the present invention. For example, as shown in FIG. 4, plasma
from the blood entering extraction channel 402 through inlet
channel 406 is skimmed and exits with sheath fluid through exit
channels 410 and 414. This process of skimming is explained in
greater detail below in connection with FIG. 7,
[0047] FIG. 5, meanwhile, shows an image of the right-most portion
of separator 400 shown in FIG. 4, as obtained by using a CCD camera
(Sensys0401E, Roper Scientific). In particular, the image of FIG. 5
illustrates plasma being skimmed from blood according to the
principles of the present invention. As shown in FIG. 5, a portion
of the blood 501 provided through inlet channel 402 (not shown)
exits through exit channel 405. Moreover, while cellular components
of blood 501 migrate to the center (as explained below in
connection with FIG. 7), cell-depleted (or cell-free) fractions of
blood 501 such as plasma 502 and 503 combine with sheath fluid 504
and 505 to exit extraction channel 400 through exit channels 404
and 406, respectively.
[0048] It will be understood by persons skilled in the art that a
membraneless separator as described herein is not intended to, nor
could it, offer sufficient discrimination between the substances it
is intended to remove and those it is intended to leave behind.
Accordingly, for example, membraneless separators as described
above will only function by themselves in the exceptional
circumstance that all the components of plasma are to be removed.
For example, a membraneless separator may be used alone when the
removal of plasma, usually not in its entirety but without
discrimination among its components, is to be removed, and the
cellular components of blood are to be left behind.
[0049] In all other circumstances, according to the principles of
the present invention, a membraneless separator will operate in
conjunction with a secondary separator that receives the sheath
fluid and, optionally, a cell-depleted (or cell-free) part of the
bloodstream. For example, to prevent the removal of macromolecules,
the secondary separator can be used to generate a stream rich in
macromolecules and free of small metabolite molecules and middle
molecules that is recycled in sheath fluid to the membraneless
separator. Thus, according to the invention, the secondary
separator regulates the operation of the membraneless separator
through the composition of the recycle stream that it returns to
the inlets for sheath fluid of the membraneless separator (as shown
in FIG. 6 and described in greater detail below). It should be
understood that the secondary separator may incorporate a variety
of means to remove solutes whose extraction removal from the
circulation (i.e., the recycle stream) is desired, and that the
invention is not limited in this manner.
[0050] One substance whose transport (i.e., removal from blood
being processed) is typically undesirable is albumin. In each pass
through an exchange device according to the invention, for example,
albumin would be removed at more than 1/4th the rate of small
solutes, and albumin (which is confined to the blood space of an
animal) would undergo perhaps 10 times as many passes as would urea
which is distributed throughout the total body water reservoir.
Thus, the fractional removal of albumin, even though its inherent
diffusivity is smaller, would exceed the fractional removal of
urea. According to the principles of the present invention,
therefore, a secondary separator (e.g., a membrane device that
permits extraction of urea and water but not albumin) may be used
to recycle albumin to the blood. In particular, the sheath fluid
received from the recycle stream will be depleted of urea and
water, but will be rich in albumin. Thus, the composition of this
stream will recruit the further extraction of urea and water but
will not recruit further extraction of albumin, given that the
difference in albumin concentration between the blood being
processed and the sheath fluid will have disappeared.
[0051] It will be understood that an important specification of how
the membraneless separator operates is the difference between the
inlet flow rate and the outlet flow rate of the sheath fluid. For
example, when these flows are equal and urea and water are removed
by the secondary separator, there will be, at first, an
insufficient transfer of water from blood to the sheath fluid to
keep up with water removal in the secondary separator. Thus the
concentration of proteins, including albumin, will rise in the
recycle stream. When this concentration has reached a sufficiently
high level, water transfer will be enhanced by a difference in
protein osmotic (oncotic) pressure between the blood and the sheath
fluid. Thus, the membraneless separator will balance its
performance to that of the secondary separator. On the other hand,
if the rate of withdrawal of sheath fluid is greater than its rate
of supply, sufficient water may be sent to the secondary separator
to keep up with its rate of water removal, but protein
concentration will rise again until a concentration difference
exists in the membraneless separator between the sheath fluid and
the blood, causing a diffusion of protein back into the
bloodstream. Once again, the membraneless separator will balance
its performance to that of the secondary separator.
[0052] For example, when the principal goal of the treatment is the
removal of highly diffusible (in general, low molecular weight)
molecules, assuming a flow of 20 ml/min flow, the contact area in
the membraneless separator will be in the range about 17 to 71
cm.sup.2. When the principal goal of the treatment is the removal
of slowly diffusible molecules (e.g., proteins and especially
immunoglobulins), the contact area in the membraneless separator
will be larger, in the range of approximately 1,700 to 7,100
cm.sup.2 (assuming a flow of 20 ml/min), and the secondary
separator will be configured to remove these molecules and to
recycle smaller molecules (unless their simultaneous removal is
desired).
[0053] FIG. 6 shows a simplified block diagram of a system 600
including membraneless separator 602 and secondary separator 604 in
accordance with the principles of the present invention. Although
not shown in detail, it will be understood that membraneless
separator 602 may be similar to those separators shown in FIGS. 3
and 4 and described above, for example.
[0054] According to the principles of the present invention, blood
that is to undergo processing is provided to (and removed from)
membraneless separator 602. Meanwhile, sheathing fluid that is
recycled by secondary separator 604 is also provided to (and
removed from) membraneless separator 602. As also shown in FIG. 6,
whenever secondary separator 604 transfers solutes to a second
fluid (e.g., dialysate), fresh dialysate connection 606 and waste
dialysate connection 608 may be used for providing fresh and waste
dialysate streams, respectively. It will be understood that
shunting of fresh fluid directly to the blood stream, as
represented by dashed line 610, is also a possibility (but not
mandatory). In general, FIG. 6 makes the role of membraneless
separator 602 clear: to equilibrate solutes of interest with the
sheathing fluid without transfer of cells.
[0055] It will be understood that secondary separator 604 may use
any of many available separation principles known to those skilled
in the art, including ultrafiltration and sorption using a wide
range of sorbents targeted to particular small and large molecules,
chemical reaction, and precipitation. Plasma diafiltration (a
variant of hemodiafiltration), for example, may also be used
according to the principles of the present invention. The following
international publications which refer to hemodiafilters are
incorporated by reference herein: WO 02/062454 (Application No.
PCT/US02/03741), WO 02/45813 (Application No. PCT US01/47211), and
WO 02/36246 (Application No. PCT/US01/45369). According to
additional embodiments of the present invention, moreover, when
low-molecular weight solutes are to be removed by plasma
diafiltration, a stream of sterile buffer is added to the blood to
allow a greater volume of fluid, with its accompanying small
molecules, to pass through the diafiltration membrane. In
conventional diafiltration, this volume may be added before or
after the diafilter. In this invention, however, it is advantageous
to add it either to the bloodstream or the recycle fluid from the
secondary separator 604, which is the primary source of sheath
fluid.
[0056] A more detailed view of a system 700 which includes
membraneless separator 702 and secondary separator 704 in
accordance with the principles of the present invention is shown in
FIG. 7. As shown in FIG. 7, separator 702 includes extraction
channel 706, inlet channels 708, 710 and 712 and exit channels 714,
716 and 718.
[0057] According to the principles of the present invention, system
700 also includes blood supply 720, and a plurality of pumps 722,
724 and 726 (which may be either manually or automatically
operated, such as by using detection and regulation techniques
described below). As shown in FIG. 7, blood supply 720 provides
blood to be processed to membraneless separator 702 through blood
inlet channel 710. It will be understood that blood supply 720 may
be a living person or other animal, for example, or may be a blood
reservoir. Blood withdrawal pump 722, meanwhile, is responsible for
removing blood from separator 702 through blood exit channel
716.
[0058] As illustrated by FIG. 7, the flow of sheath fluid (or
extractor fluid) into separator 702, through sheath inlet channels
708 and 712, is controlled by sheath fluid injection pump 724
(which preferably provides sheath fluid in equal parts to channels
708 and 712). The flow of sheath fluid out of separator 702,
through sheath exit channels 714 and 718, meanwhile, is controlled
by sheath fluid withdrawal pump 726 (which preferably draws equal
amounts of sheath fluid out of channels 714 and 718). According to
preferred embodiments of the present invention, pump 724 is a
two-chamber pump that provides sheath fluid at equal velocities
(and with substantially similar composition) to both inlet channels
708 and 712, while pump 726 is a two-chamber pump that removes
sheath fluid from exit channels 714 and 718 at equal velocities.
Moreover, it is also contemplated that pump 724 be replaced by two
pumps (not shown) for separately providing sheath fluid to inlet
channels 708 and 712, in which case the composition of the sheath
fluid entering inlet channel 708 may be substantially similar to,
or different from, the sheath fluid entering inlet channel 712.
Similarly, two pumps (not shown) can be used in place of pump 726
for the purpose of separately withdrawing sheath fluid from exit
channels 714 and 718. It is also contemplated that, in other
embodiments of the present invention, sheath fluid entering through
inlet channel 708 and exiting through exit channel 714 flows at a
different velocity than the sheath fluid entering through inlet
channel 712 and exiting through exit channel 718. It will be
understood that the invention is not limited by the particular
usage of pumps or sheath velocities described herein in connection
with the description of FIG. 7.
[0059] As explained above, a membraneless separator according to
the invention also needs one or more diverters to operate. Thus,
according to the principles of the present invention, a first
diverter 726 is formed from a portion of sheath exit channel 714
and a portion of blood exit channel 716. Moreover, a second
diverter 728 is formed using a portion of blood exit channel 716
and a portion of sheath exit channel 718. It will be understood
that, in embodiments of the present invention using more than two
layers of sheath fluid, addition diverters will be used.
[0060] In certain preferred embodiments of the invention, the
sheath fluid provided to separator 702 (from separator 704 and/or
optional sheath fluid reservoir 730) by sheath fluid injection pump
724 occupies approximately 2/3 of the cross-section of extraction
channel 706, with blood from blood supply 720 in the middle 1/3. In
this manner, each half of the blood layer in extraction channel 706
is "serviced" by one of the sheathing layers, and the sheathing
layers are traveling at an average velocity that is approximately
half that of the blood (even though the interfacial velocities of
the blood and sheathing fluids are equal). Thus, the volume of
blood and the volume of sheathing fluid that pass through the unit
in a given period of time are approximately equal. Although the
invention is not limited in this manner, it should be noted that,
in the configurations described here, efficiency drops when the
volumetric flows of the two fluids (i.e., blood and sheath fluid)
are very different from each other.
[0061] In order to cause the separation (or skimming) of all or
part of the cell-depleted component of the blood being processed,
according to various embodiments of the present invention, the
inlet and exit flows of the sheath fluid are controlled (via pumps
724 and 726, respectively) such that more sheath fluid is withdrawn
from separator 702 than is provided thereto. For example, it is
possible to skim 10% of the blood flow by running sheath fluid
withdrawal pump 726 at a rate that is 10% higher than the rate of
sheath fluid injection pump 724. It will be appreciated that, when
this is done, the blood efflux rate is determined and need not be
controlled, as it should naturally have an outflow that is 90% of
the inflow.
[0062] As explained above, when indiscriminate plasma removal is
not desired, the plasma that is skimmed from the blood using
membraneless separator 702 is processed by secondary separator 704,
which regulates the operation of separator 702 through the
composition of the recycle stream that it returns to sheath inlets
channels 708 and 712 (i.e., a recycle stream is used to limit
transport of blood components for which extraction is not
desirable). According to the principles of the present invention, a
substantial benefit arises because secondary separator 704, whether
membraneless or not, is able to achieve high filtration velocities
due to the fact that concentration polarization is limited to
proteins and does not involve cells. Moreover, because cells are
retained in the membraneless separator 702, they would see
artificial material only on its conduit surfaces, not on its
liquid-liquid contact area, with the result being a reduction in
bioincompatibilities and a reduced (or eliminated) need for
anticoagulation. Additionally, because the primary transport
surface in the system is intrinsically non-fouling, a major
deterrent to long-term or continuous operation is removed, opening
the possibility of a wearable system with the recognized benefits
of prolonged, slow exchange.
[0063] It should be understood that any operation of membraneless
separator 702 that allows the sheath exit flows to be larger than
the corresponding inlet values will induce a convective flow from
the blood stream, over and above the diffusive flow. In order to
prevent such a convective flow from carrying blood cells with it
(as would be the case if the distribution of cells in the blood
stream was uniform), it is important that cellular components of
the blood have migrated to the center of the blood stream in order
to permit significant plasma skimming. As should be appreciated by
those skilled in the art, centripetal drift of cells occurs under a
variety of flow regimes. According to the invention, therefore,
various flow conditions can be used that cause blood cells to move
away from the blood-liquid interface. For example, when blood flows
in a tube below a wall shear rate (measured as the blood-flow
velocity gradient perpendicular to the tube wall) of about 100
reciprocal seconds, this shear rate causes cellular components to
migrate the center and leave the sheath as cell-free, essentially
pure plasma. (See Goldsmith, H. L. and Spain, S., Margination of
leukocytes in blood flow through small tubes, Microvasc. Res. 1984
March; 27(2):204-22.)
[0064] It will be appreciated that long-term stability is necessary
for satisfactory operation of the microfluidic devices described
herein. For example, it is desirable to prevent inappropriate
splitting of an exit stream which, uncorrected, could result either
in loss of cells or unintended infusion of sheathing solution into
the bloodstream. Moreover, the presence of blood cells in the
sheath, or extractor fluid may also be undesirable. According to
another aspect of the present invention, therefore, on-board
electronics and photonics (not shown), which are common features of
chip-based microfluidic devices, may be used. In particular, such
electronics or photonics could be used to regulate system 700
(i.e., to introduce flow changes) with an electrically activated
device (e.g., a piezoelectric valve) that is mounted on the same
plate, or "chip," on which separator 702 is located.
[0065] According to one embodiment of the invention, for example,
very low concentrations of cells would be permitted and monitored
(e.g., before or after the sheath fluid being provided to secondary
separator 704) by using any suitable detector, such as a photo
detector. An ultramicroscope (a light-scattering device that is
particularly sensitive to the presence of dilute particles) is one
example of a photo detector which can be used. Based on this
monitoring, flow corrections that would provide the system with
long-term stability can be made which include, for example,
adjusting the blood-sheath fluid interface. In particular, by
adjusting the flows to separator 702 to reposition the interface,
desired components can be retained in the blood. For example, when
an excessive number of blood cells is present, the flow of blood
could be decreased (or the flow of extractor fluid increased) in
order to shift the blood-sheath fluid interface accordingly.
[0066] Additionally, according to another aspect of the invention,
on-board electronics can be used to protect against the type of
flow imbalances that might cause large blood losses in one
direction or massive hypervolemia in the other direction, which are
naturally prevented when a membrane is present but which may occur
in a membraneless device. It will be understood by those skilled in
the art this type of detection and regulation may also be used with
in conjunction with the other embodiments of the present invention
described above.
[0067] As explained above, in all membraneless contact
configurations, the fluids (e.g., blood and sheath fluid) must flow
in the same direction. In particular, any flow in opposite
directions would disrupt the blood-fluid interface and induce
undesirable advection. Moreover, since the fluids must flow in the
same direction, the most that can be accomplished in one
membraneless unit according to the invention is the equilibration
of the sheath and blood streams (which, according to Loschmidt's
formula provided above, means that if the sheathing fluid is flowed
at the same rate as blood, the extraction E of a solute cannot
exceed 1/2). In other words, if the two flows are equal, at most
half of any solute can be transferred. Moreover, while greater
flows permit larger fractions, E, of a solute to be removed, they
require higher circulation rates to the secondary separator and
thus force processing of solutes at lower concentrations, which is
generally undesirable. Therefore, it is generally desirable for
these flows to be nearly equal, within at least a factor of 2 or
3.
[0068] This limitation on extraction can be largely overcome,
however, by the configurations shown in FIGS. 8 and 9 and described
below which achieve the effect of opposing flows (counterflow) by
the juxtapositions of modules. In particular, low extraction
efficiency can be overcome by more sophisticated layouts of a
microfluidic system such that flows are concurrent in each unit of
the system, but the overall flow approaches countercurrency in
pattern and efficiency.
[0069] According to the invention, subdivision of a given, desired
contact area into n units each connected to the other in a
countercurrent manner, even though the flow within them is
concurrent, is used to allow extraction efficiency to rise
according to the formula provided above. Thus, if an area were
divided into four units, for example, and each had an extraction
efficiency of 50%, the composite unit would have an efficiency of
0.8 or 80%.
[0070] FIG. 8 shows the configuration of a system 800 according to
the invention in which the total area of contact is partitioned
into three sub-units 802, 804 and 806 (i.e., n=3). In operation,
blood to be processed is first provided to sub-unit 802, then
passes through sub-unit 804, and finally, exits out of sub-unit
806. The sheath fluid to be used in system 800, on the other hand,
is first provided to sub-unit 806 (at this point, the sheath fluid
has no blood components). The sheath fluid exiting sub-unit 806 is
next provided to sub-unit 804, and after exiting sub-unit 804, is
provided to sub-unit 802. Thus, assuming each unit has an
extraction efficiency of 50%, the overall extraction efficiency of
the composite unit, E.sub.O, is equal to 0.75 or 75%. Accordingly,
it becomes possible, at equal flows, to remove 75% rather than only
50% of the solute of interest. In will be understood that the
extraction efficiency approaches 1.0 or 100% as the number of small
units approaches infinity. Persons skilled in the art will
appreciate that, although not shown, the sheath fluid exiting
sub-unit 802 may be provided to a secondary separator as described
above. Moreover, while three sub-units 802, 804 and 806 are shown
in FIG. 8, it will be understood that any number of sub-units
(e.g., 2, 4, 5, etc.) may be used in system 800, all of which may
be easily introduced on a master chip fabricated according to well
known techniques for the general fabrication of microfluidic
devices.
[0071] FIG. 9 shows another example of a system 900 using sub-units
according to the principles of the present invention. In
particular, FIG. 9 shows two flow patterns 902 and 904 that would
be superimposed on each other in a single cartridge. For example,
the top could represent blood, while the bottom could represent an
extractor fluid (e.g., dialysate). As shown in FIG. 9, sheath fluid
flows through sub-unit 906 prior to flowing through sub-unit 908.
In this manner, with sufficient contact area, the fraction of
material in the blood layer that is extracted will be equal to 2/3
or 67%.
[0072] Persons skilled in the art will appreciate that many
different fabrication techniques can be used in accordance with the
principles of the present invention. In recent years, controlled
fluid movement and transport among fluids has been achieved in very
small channels and at very low rates of flow largely for the
purpose of assaying the contents of a minute fluid sample in order
to determine, for example, the catabolite concentration in the
blood. These devices have been enabled by recently developed
microfabrication methodologies. The Holy Grail has been the
development of a "Lab on a Chip," in which several sequential
analytical processes are conducted on a single chip that may be,
for example, one square centimeter in area. Transport of a chemical
or biochemical sample from one process to another and on and off
the chip itself requires fluid handling capabilities, and thus,
this enabling technology is commonly called "microfluidics."
Microfluidics is essential for nearly all on-chip applications. The
synthesis of chemicals in microfluidic geometries is an application
that is perhaps closer in concept to the scope of this disclosure
because of the need to process a relatively larger amount of fluid.
Synthesis includes, perforce, the separations needed between the
steps of a chemical reaction sequence. While the aims of
synthesizers differ from ours, and embrace some issues that we do
not now see as pertinent, all of this work, reported and emergent,
is of interest. Specifically, the present invention embraces some
of the fabrication techniques and experimental methods developed
for the fabrication and characterization of microfluidic device
structures, to define upwardly scalable transport to and from
blood.
[0073] According to the invention, moreover, microchannel
structures for flow experiments may be formed by a
rapid-prototyping technique. For example, the required structures
may be realized in PDMS (silicone) resin by replica-molding from
master structures created in thick negative photo resist (SU-8) by
optical lithography. Commercially available, standard grade
mixtures of EPON SU-8 photo resist, SU-8-5 (52% solids), SU-8-25
(63% solids), SU-8 50 (69% solids) and SU-8 100 (73% solids), for
example, may be spun onto Si wafer substrates at a speed of
rotation that depended on the film thickness needed, yielding films
that were 10 to 300 .mu.m thick. For example, SU-8 50 spun at 1100
rpm yields a 100 .mu.m film. Prior to exposure, moreover, the spun
layer is preferably baked on a precisely leveled hot plate at
95.degree. C. for a time that is dictated by the film thickness
(ranging from minutes to hours). These samples are then allowed to
cool before further processing. Post-bake exposure, meanwhile, can
be done using a direct laser writing system. The photolithographic
setup consists of an Ar-ion laser (wavelength .lamda.350 nm),
focusing optics, and a computer controlled sample stage. The
movement of the stage along all three axes (x, y, z) is achieved by
stepping motors. Desired master patterns were created by
translating the samples underneath the focused laser beam to expose
the outline, and then scanning across the interior so that the
intended micro channel was fully exposed. Dynamical focus
correction or the sample tilt with respect to the scanning laser
beam was the done by on-the-fly adjustments of the distance between
the focusing lens and the sample stage. In a preferred embodiment,
this exposure is carried out at 95.degree. C. for 15 min.
Development, meanwhile, can be carried out in a commercial SU8
developer, again for a time based on film thickness (with the
sample being lightly stirred during development). Patterns created
in SU-8, meanwhile, are used as molding masters for replication in
PDMS. PDMS is prepared from a mixture of PDMS precursor and curing
agent (Sylgard 184 kit, Dow Corning) in a 10:1 ratio by weight.
Before curing, the mixture is placed in vacuum to evacuate bubbles
formed during mixing. It is then poured over the SU-8 master, which
had been previously coated with a thin layer (.about.50 nm) of
chromium to improve the release of the PDMS casting, after curing.
Curing is done at 70.degree. C. for approximately twelve hours.
Once the SU-8 film is spun, pre-baked and cooled as described
above, a Karl Zeiss MJP3P Contact Mask Aligner can be used for
exposure, together with standard chromium masks or transparency
masks depending on the resolution required. The films are then
post-baked, and developed in the manner outlined in the previous
section. The same pattern transfer technique is used to produce
PDMS replicas.
[0074] It is apparent to those skilled in the art that many
advantages may be provided in the various embodiments of the
present invention described above. For example, the devices,
systems and methods according to the principles of the present
invention are capable of diffusing various blood components having
different sizes, including `small` molecules, `middle` molecules,
macromolecules, macromolecular aggregates, and cells, from a blood
sample to the extractor fluid. This ability is particularly
important considering the fact that different treatments require
the removal of different sized particles. For example, in dialysis,
one may desire to remove molecules of low molecular weight, while
in the treatment of acute liver failure, both small and
intermediate-sized molecules are to be removed. In therapeutic
apheresis, meanwhile, one generally wishes to remove selected
protein macromolecules (e.g., immunoglobulins), while in the
treatments for fulminating sepsis, it is toxins of intermediate
molecular weight that one generally desires to remove. On the other
hand, in proposed anti-viral treatments, one wishes to remove free
viral particles, while in the treatment of congestive heart
failure, one simply wishes to remove water.
[0075] It should also be apparent that a device or system according
to the invention may be used to process the blood of a single
individual for the purpose of treating any of a large number of
disease states. For example, therapies according to the invention
may be used in the treatment of acute renal failure, acute liver
failure, high antibody levels in myasthenia gravis and other
autoimmune diseases. Additional uses include, for example, the
removal by either precipitation or sorption of LDL in homozygous
hyperlipidemia, in addition to the removal of malignant sepsis or
fluid in cases of congestive heart failure, for example. The
invention may also be used to aid in the reduction of viral burdens
in AIDS patients, as well as for treatment of patients requiring
other types of blood purification. Patients with diabetes, patients
that have suffered a drug overdose, patients that have ingested a
poison, patients suffering from renal failure, patients suffering
from acute or chronic liver failure, or patients that have
Myasthenia gravis, lupus erythematosis, or another autoimmune
disease may also benefit from the devices and systems of the
present invention. For example, while an exchange device according
to the invention is not a cure for diabetes, it can be useful in
the amelioration one or more symptoms of diabetes. Moreover, the
device or system of the invention could be useful in clearing the
blood of IgG molecules or other molecules, which are causative of
an autoimmunity disorder. Additionally, the device or system of the
invention can be used in acute dialysis or for extended dialysis.
One skilled in the art will also appreciate that patients (or
animals, in the case of veterinary use of the present invention)
suffering from disorders, diseases and syndromes not listed herein
may nonetheless be included in the patient pool intended for the
device and system according to the invention.
[0076] Additionally, because the membraneless devices and systems
described above have a small need for supporting machinery, and may
be expected to be much smaller, to avoid high cell concentrations
and membrane contact, and to operate throughout at low rates of
shear, they are especially compatible with cognate processes. In
one embodiment, a wearable (or at least portable) system according
to the invention can run between 20 and 24 hours per day at a flow
rate of about 20 cc/min, for example. The patient could then have,
for example, 4-5 hours each day without the device in place which
could be used for personal hygiene (e.g., showers or baths), sports
activities, or other activities not amenable to the small system
being worn or used. The invention thus addresses a problem
recognized by the dialysis community (i.e., the negative side
effects such as physical exhaustion, thirst, etc. associated with
an episodic dialysis schedule), for which daily or nocturnal
hemodialysis is not always a sufficient alternative. In particular,
the invention described herein allows the patient to move about in
a normal manner (e.g., go to work, school, home, etc.) while being
subject to ongoing dialysis.
[0077] In addition to the treatment of various disease states, a
device or system according to the invention may also be used for
extracting blood components that are useful in treating others, as
well as for purposes of studying the processes by which molecules
and cells segregate and diffuse in blood. For example, it is known
to those skilled in the art that diffusion of individual molecular
species in blood may not occur independently and may not depend on
size in the simple manner dictated by the Stokes-Einstein equation.
Moreover, many solutes may partition into multiple forms: free, in
complexes, bound to plasma protein, bound to cell-surface moieties,
or as intracellular solutes. Relative to the rate of diffusion of
the solute, its different forms may or may not be in local
equilibrium. These phenomena are likely obscured when a membrane is
present because it slows and controls overall transfer rates.
Therefore, a membraneless device or system according to the
invention can be a useful scientific tool to study these phenomena
and a system in which rates are raised enough that partitioning may
set limits on how much and how quickly a solute can be removed. A
particular example is bilirubin bound to albumin. Another example
is inorganic phosphorous which exists as partially ionized salts,
as two anionic forms in plasma and in several intracellular
forms.
[0078] Persons skilled in the art will also appreciate that the
present invention can be practiced by other than the described
embodiments, which are presented for purposes of illustration and
not of limitation, and that the present invention is limited only
by the claims that follow.
* * * * *